common-defs.h: include <stdarg.h> before <stdio.h>

common-defs.h: include <stdarg.h> before <stdio.h>

When trying to build gdbserver on ppc-lynx178, the compiler reports
while trying to compile gdbserver/ax.c that vsprintf is not declared.
Looking at my C99 reference manual (a draft), I see the following
synopsis:

    #include <stdarg.h>
    #include <stdio.h>
    int vsprintf(char * restrict s, [etc]);

Looking at stdio.h on LynxOS-178, if found where vsprintf gets
declared:

    #if defined(__varargs_h) || defined(__stdarg_h) \
            || defined(_VARARGS_H) || defined(_STDARG_H)
    extern int vsprintf             _AP((char *, const char *, va_list));
    #endif

Digging further, I noticed that common-defs.h, which is included
via server.h, includes stdarg.h after including stdio, explaining
why vsprintf does not get declared in this case.

This patch fixes the problem by including stdarg.h before stdio.h.

gdb/ChangeLog:

        * common/common-defs.h: Move <stdarg.h> #include ahead of
        <stdio.h> #include.

Tested on x86_64-linux.